1. Field of the Invention
The present invention relates to a wireless communication system, more particularly to a method of performing a random access procedure in a multiple component carrier system.
2. Related Art
Wireless communication systems are widely deployed in order to provide various kinds of communication services, such as voice and data. In general, the wireless communication systems are multiple access systems which can share available system resources (e.g., bandwidths and transmission power) and support communication with multiple users. The multiple access systems can include, for example, a Code Division Multiple Access (CDMA) system, a Frequency Division Multiple Access (FDMA) system, a Time Division Multiple Access (TDMA) system, an Orthogonal Frequency Division Multiple Access (OFDMA) system, and a Single Carrier Frequency Division Multiple Access (SC-FDMA) system.
In a common wireless communication system, although the bandwidth of uplink and the bandwidth of downlink are differently set up, only one carrier is taken into consideration. Even in 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), the number of carriers constituting uplink and downlink is 1, and the bandwidth of uplink and the bandwidth of downlink bandwidth are in general symmetrical to each other on the basis of a single carrier. However, it is not easy to allocate a large frequency bandwidth except some regions in the world. Accordingly, in order to efficiently use small sliced bands, a carrier aggregation technique for producing the same effect as that a logically large band is used by binding a plurality of physically non-continuous bands in the frequency domain is being developed. The carrier aggregation technique includes, for example, a technique in which system bandwidth of 100 MHz is supported by using multiple carriers.
A mobile station (MS) shall perform random access procedure to gain access to a network. The random access procedure is accomplished through a Random Access Channel (RACH), i.e., an uplink transmission channel. The MS transmits an initial control message (i.e. preamble) to the network through the RACH. The aim of performing the random access procedure is to obtain time synchronization between the MS and the network or to obtain radio resources for uplink transmission. There are two kinds of random access procedures. In non-contention-based random access procedure, a dedicated identifier for identifying an MS is allocated. Therefore, random access procedure can promptly be completed since contention between MSs does not occur. But resources for the dedicated identifier are needed. On the contrary, in contention-based random access procedure, an MS initiates the random access procedure without the dedicated identifier. Identifier for the MS is allocated during the random access procedure to resolve contention between MSs.
If carrier aggregation is used in the contention based random access procedure, it can be assumed that multiple uplink carriers and multiple downlink carriers are aggregated and random access resources are allocated to the multiple uplink carriers. Under this assumption, an MS can transmit multiple preambles on the multiple uplink carriers respectively. It is not clear which downlink carrier the MS needs to monitor among the multiple downlink carriers to receive its identity for contention resolution of the random access.